Ozone in the atmosphere prevents most harmful UV radiation from reaching the biosphere. About 90 to 95% of atmospheric ozone is found in the stratosphere; the remaining 5 to 10% is in the troposphere. Most of the stratospheric ozone is produced by photochemical reactions in equatorial regions; at high latitudes, there is less photochemical ozone production and much of the stratospheric ozone is imported from low latitudes by the Brewer-Dobson circulation. This diabatic circulation also distributes ozone to lower altitudes in the high latitude regions, where, owing to a longer photochemical lifetime, it accumulates. For these reasons, total column ozone tends to exhibit global maxima near the poles. The atmospheric circulation varies seasonally, and oscillations in the circulation patterns explain some of the natural spatial, seasonal, and annual variations in the global total ozone distribution. In the Northern Hemisphere, the maximum total column ozone usually occurs in spring and the minimum in autumn. Solar activity also causes small fluctuations in total column ozone in phase with the solar cycle.

Fig. 5.2. Spring ozone depletion over the Antarctic and the Arctic between 1979 and 2002. (Source: ACIA)

In addition to natural ozone production and destruction processes[1], stratospheric ozone is destroyed by heterogeneous chemical reactions involving halogens, particularly chlorine and bromine, which are derived from chlorofluorocarbons (CFCs) and other ozone-depleting substances. In the presence of solar radiation, extremely low stratospheric temperatures facilitate ozone depletion chemistry. Thus, ozone depletion can occur in relatively undisturbed polar vortices (see section 5.2.2, Box 5.1) with the return of sunlight in early spring. The fundamental processes governing ozone levels over the Arctic and Antarctic are the same, however, relative rates of production and destruction can differ. Low temperatures within the stable Antarctic vortex and the presence of ozone-depleting gases have led to an area of large-scale ozone depletion, the “ozone hole”, which has been observed every spring since the 1980s. In contrast, the arctic polar vortex is less stable, resulting in arctic air masses that are on average warmer than air masses over the Antarctic. However, chemical ozone depletion has been observed over the Arctic during springs when temperatures in the arctic stratosphere were lower than normal. The decreases over the Arctic and Antarctic have both been sizeable (Fig. 5.2), although climatological spring ozone levels over the Arctic tend to be higher than those over the Antarctic, so that total column ozone after depletion events is higher in the Arctic than in the Antarctic. The depletion observed over the Antarctic in spring 2002 was not as severe as in previous years, but this was due to exceptional meteorological conditions and does not indicate an early recovery of the ozone layer.

Since the late 1980s, much attention has been directed to studying ozone depletion processes over the Arctic. Arctic ozone levels have been significantly depleted in the past decade, particularly during the late winter and early spring (seasons when pre-depletion ozone levels were historically higher than at other times of the year). Several studies[2] have focused on both the chemical and dynamic factors contributing to this depletion. These factors have combined to change the overall concentrations and distribution of ozone in the arctic stratosphere[3], and the observed changes have not been symmetric around the North Pole. The greatest changes in ozone levels have been observed over eastern Siberia and west toward Scandinavia.

Ozone depletion can increase the level of UV radiation reaching the surface. These increased UV doses, particularly when combined with other environmental stressors, are very likely to cause significant changes to the region’secosystems. Ozone depletion has been greatest in the spring, when most biological systems are particularly sensitive to UV radiation. The depletion has not been constant over time: very strong ozone depletion has been observed in some years while very little depletion has been observed in other years.

Transport of low-ozone air masses from lower latitudes can result in a few days of very low ozone and high UV radiation levels[4]. This transport of low-ozone air masses is often observed in late winter or early spring and is likely to have occurred naturally for decades. Climate change is likely to change transport patterns and is therefore likely to alter the frequency and severity of these events[5].

Halogens and Trace Gases (5.2.1)

Chlorine and bromine compounds cause chemically induced ozone depletion in the arctic stratosphere[6]. The source gases for these halogens are predominantly anthropogenic[7] and are transported to the polar stratosphere over a period of 3 to 6.5 years[8]. In the stratosphere, the source gases are converted through photolysis and reaction with the hydroxyl radical to inorganic species of bromine, chlorine, and fluorine. The halogens are normally present as reservoir species (primarily hydrogen chloride – HCl, chlorine nitrate – ClONO2, and bromine nitrate), which are efficiently converted into photochemically active species in the presence of sulfate aerosols or polar stratospheric clouds[9]. Subsequently, in the presence of sunlight, reactive compounds (e.g., chlorine monoxide, bromine monoxide) are formed that react with and destroy stratospheric ozone in catalytic cycles.

The concentrations of chlorine measured in the stratosphere correspond well with the concentrations of CFCs and related gases that have been measured in the troposphere[10]. From the mid-1980s to the early 1990s, the atmospheric chlorine concentration increased approximately 3 to 4% per year[11], while between 1995 and 1997, the rate of stratospheric chlorine increase was estimated at 1.8±0.3% per year[12]. An analysis of longterm trends in total column inorganic chlorine through 2001, based on 24 years of HCl and ClONO2 data, showed a broad plateau in inorganic chlorine levels after 1996[13]. Some uncertainty remains concerning the time lag between reductions in emissions of chlorine-containing compounds at the surface and chlorine concentrations in the stratosphere[14], although this lag is thought to be between 3 and 5 years on average. Other studies report an estimated total organic bromine growth rate of 2.2% per year[15], although errors in the experimental method make the stratospheric bromine mixing ratios more difficult to determine. More recently, Montzka et al.[16] reported that total organic bromine amounts in the troposphere have decreased since 1998.

Changing concentrations of the trace gases nitrous oxide (N2O), methane (CH4), water vapor, and carbon dioxide (CO2) directly affect ozone chemistry and also alter local atmospheric temperatures by radiative cooling or heating, influencing the reaction rates of ozone depletion chemistry and the formation of ice particles. All of these gases emit radiation efficiently to space from the stratosphere (although CO2 and water vapor are the most important), so increases in the abundances of these gases are very likely to lead to stratospheric cooling. In the polar regions, this cooling is very likely to lead to ozone depletion through heterogeneous chemistry. Lower temperatures facilitate the formation of polar stratospheric cloud particles, which play a role in transforming halogens to reactive compounds that can destroy ozone very rapidly. Small changes in temperature have been shown to have a significant effect on ozone levels[17].

The trace gases N2O, CH4, and water vapor are also important chemically. In the stratosphere, CH4 acts as an important source of water vapor and is also a sink for reactive chlorine. In addition, stratospheric water vapor is an important source of hydrogen oxide radicals, which play an important role in ozone destruction. Evans et al.[18], Dvortsov and Solomon[19], Shindell[20], and Forster and Shine[21] have studied the effects of water vapor on homogeneous chemistry. Their model results suggest that increases in water vapor reduce ozone levels in the upper stratosphere, increase ozone levels in the middle stratosphere, and reduce ozone levels in the lower stratosphere. Ozone levels in the lower stratosphere dominate total column ozone, and the model results differ most in this region. In the simulations of Evans et al.[22], reductions in lower-stratospheric ozone levels occur only in the tropics when water vapor increases, while in the other simulations, the reductions extend to the mid-latitudes or the poles. The models of Dvortsov and Solomon[23] and Shindell[24] projected a slower recovery of the ozone layer as a result of increased stratospheric water vapor, and a 1 to 2% reduction in ozone levels over the next 50 years compared to what would be expected if water vapor did not increase.

Water vapor affects heterogeneous chemistry by enhancing the formation of polar stratospheric clouds (PSCs). This effect may be much more important than the relatively small impacts of water vapor on homogeneous chemistry. Kirk-Davidoff et al.[25] projected a significant enhancement of arctic ozone depletion in a more humidatmosphere. Much of this projected effect is based on the radiative cooling of the stratosphere assumed to be induced by water vapor, a value that is currently uncertain. A smaller value would imply a reduced role for water vapor in enhancing PSC formation. Even using a smaller cooling rate, however, the impact on ozone is likely to be large, as the ~3 ºC cooling of the stratosphere projected to occur if CO2 concentrations double is of comparable magnitude to the cooling that would be caused by a water vapor increase of only ~2 parts per million by volume (ppmv). Although precise quantification of radiative forcing due to water vapor is difficult, an estimate by Tabazadeh et al.[26] suggests that an increase of 1 ppmv in stratospheric water vapor (with constant temperature) would be equivalent to a ~1 ºC decrease in stratospheric temperature and would cause a corresponding increase in PSC formation. This comparison suggests that the radiative impact of water vapor is larger than its effects on chemistry or microphysics. Given the potential for atmospheric changes in the Arctic, and the large ozone losses that could result from a slight cooling[27], it is important both to understand trends in stratospheric water vapor, and to resolve differences in model projections of the radiative impact of those trends.

Changing concentrations of the trace gases N2O and CH4 may also affect ozone levels. Nitrous oxide breaks down to release nitrogen oxide radicals, which are extremely reactive and play an important role in ozone chemistry. Increases in CH4 concentrations lead to an increase in hydrogen oxide radicals but at the same time increase the sequestration of chlorine radicals into HCl. The effects of increases in these gases on ozone depletion are thought to be relatively small[28], although a recent study by Randeniya et al.[29] suggests that increasing concentrations of N2O may have a larger impact than previously thought.

Arctic Ozone Depletion and Meteorological Variability (5.2.2)

Partitioning the transport and chlorine chemistry contributions to arctic ozone variability is a subject of much discussion[30]. The degree of ozone depletion in the Arctic depends strongly on airtemperatures and PSC formation. Several methods have been used to estimate the total column ozone depletion in the arctic polar vortex based on meteorological measurements[31], and comparisons between the different studies show good agreement[32]. Since 1988–1989, three winters (1994–1995, 1995–1996, and 1999–2000) have had particularly low stratospheric temperatures and were characterized by PSC formation in both the early and late parts of the season[33]. Some of the most severe arctic ozone losses (up to 70% at 18 kilometers (km) altitude) were observed during those winters[34].

Chipperfield and Pyle[35] used models to investigate the sensitivity of ozone depletion to meteorological variability, chlorine and bromine concentrations, denitrification, and increases or decreases in stratospheric water vapor. Although the models tended to underestimate observed rates of arctic ozone depletion, their results agreed at least qualitatively with empirical estimates of ozone depletion, which suggest that substantial arctic ozone depletion is possible when both early and late winter temperatures in the stratosphere are extremely low. Cold early winters or cold late winters alone are not enough to produce extensive ozone depletion, but can still cause depletion to occur. During the winters of 1993–1994 and 1996–1997, temperatures in the arctic stratosphere were very low in late winter compared to earlier in the season. Ozone losses at specific altitudes during these years were of the order of 40 to 50%[36].

Dynamic processes dominate the short-term (day-to-day) variability in winter and spring total column ozone at mid- and high latitudes. Local changes in total column ozone of the order of 100 Dobson units (DU) have been frequently reported[37] and are linked to three main transport processes:

A shift in the location of the polar vortex leads to changes in total column ozone, because the polar vortex air masses are characterized by low ozone levels compared to air masses outside the vortex.

Tropical upper-tropospheric high-pressure systems moving to higher latitudes cause an increase in the height of the tropopause at those latitudes, and thus a reduction in the overall depth of the stratospheric air column, as a result of divergence, resulting in ozone redistribution and a decline in total column ozone[38].

Tropical lower-stratospheric or upper-tropospheric air masses may be mixed into the stratosphere at higher latitudes. Referred to as “streamers”[39], these phenomena introduce lower ozone content to the high-latitude air masses.

These three transport processes are not independent and can occur simultaneously, potentially increasing total column ozone variability.

The occurrence of ozone minima or ozone “mini-holes” at northern mid- and high latitudes caused by tropopause lifting (process 2) exhibits high interannual variability. James[40] found no detectable trend in mini-hole occurrences using Total Ozone Mapping Spectrometer (TOMS) satellite data for the period from 1979 to 1993. However, an analysis of satellite data by Orsolini and Limpasuvan[41] found an increase in the frequency of ozone mini-holes in the late 1980s and early 1990s. The increase may be linked to the positive phase of the North Atlantic Oscillation (NAO; see section 2.2.2.1), which displaces the westerly jet to higher latitudes, allowing pronounced northward intrusions of high-pressure systems (processes 2 and 3). A similar link between the NAO and the frequency of ozone mini-holes has been found in ground-based measurements[42].

Coupled chemistry-climate models are currently able to simulate these meteorological phenomena[43]. Stenke and Grewe[44] compared simulations from a coupled chemistry-climate model with TOMS data and showed that ozone minima were fairly well represented in the simulations. Such simulations suggest that the processes affecting PSC formation can significantly increase chemical ozone depletion, leading to mini-hole occurrences or other substantial ozone minima.

Large-scale Dynamics and Temperature (5.2.3)

The Arctic is highly affected by atmospheric processes, and mid- and high-latitude dynamics can play an important role in arctic ozone depletion. The Northern Hemisphere is characterized by large landmasses and several high mountain ranges at middle and high latitudes. These geographic features generate planetary scale atmospheric waves that disturb the northern polar vortex. As a result, the polar vortex tends to be less stable and less persistent over the Arctic than over the Antarctic. Ozone depletion over the Arctic has therefore been less severe than that over the Antarctic, but is still greater than the depletion observed at tropical or mid-latitudes. Ozone depletion in the Arctic is characterized by large interannual variability, depending largely on the strength of the polar vortex and on air temperatures within it. During years when the polar vortex was especially strong, substantial (up to 40%) total column ozone depletion was observed[45].

Changes in the dynamics of the stratosphere play a role in long-term trends as well as in inter- and intraannual variability in arctic ozone levels. The stratospheric circulation determines how much ozone is transported from the lower-latitude production regions, as well as the extent, strength, and temperature of the winter polar vortex. The variability of polar vortex conditions is strongly influenced by fluctuations in the strength of the planetary-wave forcing of the stratosphere. There is evidence from both observations and modeling studies that long-term trends in arctic ozone levels are not solely driven by trends in halogen concentrations, but are also a function of changes in wave-driven dynamics in the stratosphere[46]. During years in which planetary waves penetrate effectively to the stratosphere, the waves enhance the meridional Brewer-Dobson circulation, which brings more ozone from the low-latitude middle and upper stratosphere to the polar region and then down to the arctic lower stratosphere. At the same time, the planetary waves are likely to disrupt the polar vortex, reducing the occurrence of temperatures low enough for PSC formation. Increased planetary-wave activity is thus highly correlated with greater ozone levels, but projections of future wave forcing remain uncertain[47].

Extremely low stratospheric temperatures (below 190 Kelvin) in the polar regions can lead to the formation of PSCs (Box 5.1). Polar stratospheric clouds contribute significantly to ozone chemistry, leading to accelerated ozone destruction. Over the Antarctic, stratospheric temperatures are routinely lower than these thresholds every spring. Over the Arctic, stratospheric temperatures are often near these critical temperature thresholds, such that during periods when the temperatures are slightly lower than average, accelerated ozone depletion is observed, while during periods when the temperatures are slightly higher than average, ozone levels can appear climatologically normal. Current climate models suggest that stratospheric temperatures are likely to decrease in the coming decades as a result of increasing atmospheric concentrations of greenhouse gases, thus, it is likely that there will be more periods when accelerated ozone destruction could occur. The combination of dynamics, interannual variability, and the coupling between chemistry and radiative forcing makes projecting future arctic stratospheric temperatures and ozone depletion extremely challenging.

Box 5.1.The polar vortex and polar stratospheric clouds

Winter and early spring ozone levels in the Arctic are influenced by the polar vortex, a large-scale cyclonic circulation in the middle and upper troposphere.This circulation keeps ozone-rich mid-latitude air from reaching the vortex region and can also lead to very cold air temperatures within the vortex.
Cold temperatures allow the formation of polar stratospheric clouds (PSCs), which play two important roles in polar ozone chemistry. First, the particles support chemical reactions leading to active chlorine formation, which can catalytically destroy ozone. Second, nitric acid removal from the gas phase can increase ozone loss by perturbing the reactive chlorine and nitrogen chemical cycles in late winter and early spring[48]. As the stratosphere cools, two types of PSCs can form. Type-1 PSCs are composed of frozen nitric acid and water and form at temperatures below 195 K. At temperatures below 190 K, Type-2 PSCs may form.Type-2 PSCs are composed of pure frozen water and contain particles that are much larger than the Type-1 PSC particles. Both types of PSCs occur at altitudes of 15 to 25 km and can play a role in ozone depletion chemistry, although Type-2 PSCs are quite rare in the Northern Hemisphere.

^ WHO, 2002. Global Solar UV Index: A Practical Guide. A joint recommendation of the World Health Organization, World Meteorological Organization, United Nations Environment Programme, and the International Commission on Non-Ionizing Radiation Protection.